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Timing Circuit

Discussion in 'Electronic Repair' started by [email protected], Jan 29, 2006.

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  1. Guest

    I need to design a circuit that I can tell it to start (digitally), and
    then it will tell me when 10 minutes have passed. I just need to be
    pointed in the right direction. What's the difference bewteen a
    timer/clock/counter. I have a good understanding of circuit theory but
    does anyone know of a good IC that can be surface mounted (and is
    cheap) that I shoulod use. Any help or tips are greatly appreciated
     
  2. Arfa Daily

    Arfa Daily Guest

    All depends on how accurate that you need to be really. A simple timer
    circuit - the CMOS version of the good old 555 is quite capable of this sort
    of delay - could be set up fairly precisely in monostable mode, to do 10
    minutes or thereabouts, say give or take a few seconds.

    If you really need an accurate 10 minutes, then you need to use a counter.
    You would start with a clock generator, probably a standard watch crystal at
    32.768kHz, then divide it down to get your 10 minute interval. If you don't
    want to divide as far, you can start with a lower frequency, but it's
    usually convenient to use a ' binary ' frequency. A 16 bit counter on the
    end of a 32k clock, will get you down to a second. You then need a further
    10 bits of counter and a bit of AND-ing to get you to the 10 minutes.

    You could also pick a ' custom ' frequency for the clock to suit your final
    time, which saves the gates at the end.

    Horses for courses really

    Arfa
     
  3. Mr Fixit

    Mr Fixit Guest

    buy a cheap cooking timer
    http://www.maplin.co.uk/Module.aspx?ModuleNo=36130&doy=search
     
  4. Arfa Daily spake thus:
    Since it looks like 32.768 kHz is the lowest-frequency oscillator
    available (checked both Digikey & Jameco), I'd say all you need would be
    one of them (a complete oscillator, not a crystal) and 2 counter chips
    (like the 74HCT4040), one set to divide/16 as you explained, the other
    /10. This would give you a TTL pulse at the end of the countdown cycle.
     
  5. James-

    A simple timer might consist of a resistor charging a capacitor, with a
    level detector used to determine when the desired time has been reached.
    A more sophisticated timer would use flip-flops to count pulses, with
    logic gates set to output a pulse when the state of the flip-flops reaches
    a predetermined value.

    A "counter" usually refers to an arrangement where counting is done only
    during a precise period called the timebase. The result is counts per
    unit time, usually expressed in Hertz (Hz), or counts per second. A
    free-running counter would continue counting until told to stop, and the
    results would just be the number of pulses received since the count was
    started.

    There is an IC that might get you started, the NE-555. Other sources make
    it, but always with the "555" number.

    This chip can be configured to start timing on command, and charge-up a
    capacitor through a resistor, until a preset time has past. At that point
    it should reset and generate an output pulse or level shift.

    The trouble with this approach is that it is best for short time delays.
    Ten minutes is a relatively long time, so circuit leakage may cause the
    delay to be longer than what you would calculate based on the R-C time
    constant.

    Another approach to timing, is to count a clock, such as the NE-555 being
    used as an oscillator or using the 60 Hz powerline frequency. The counter
    would be started by your digital signal. When the counter reaches a
    predetermined value, it should reset itself and generate an output pulse
    or level shift. This method is better for long delays, and is less
    affected by circuit tolerances.

    Does this help?

    Fred
     
  6. If you use the CMOS version - usually 7555 - the low current allows times
    of much more than 10 minutes.
     
  7. The problem is that you end up needing absurd component values - ten
    minutes delay would need 1.2Mohms and 470mf, for example, and the
    latter is likely to be inaccurate and probably variable.

    You can use two of them (or a 556), or there used to be a chip that
    would work up to many hours delay, the number of which I cannot
    recall; I think it was a TDA- something.

    --

    rgds
    LAurence

    ....Enter any 12-digit prime number to continue.
    ---*TagZilla 0.059* http://tagzilla.mozdev.org
     
  8. Laurence Taylor spake thus:
    Why would you want to mess with a 555 at all? Not precise, and more
    complicated. I already posted a possible solution using an oscillator
    (1MHz) and two 74HCT4040 chips, which will give you an *exact* time and
    uses fewer components. (Like no resistors or capacitors, except for
    maybe one 0.01 uf bypass cap.)


    --
    If the United States government, with all its capacity to collect
    and interpret information, did not see Hamas doing very well in the
    Palestinian election in the wake of these other Islamist victories,
    then it is either willfully blind or totally incompetent—-
    and neither possibility is a very comforting thought.

    - Rami G. Khouri, editor at large of the Beirut-based _Daily Star_
     
  9. Guest


    Well letcher laurence....
    Does everyone know that you are a pedophile?
    laurence likes to **** children.....
    alt.support.boy-lovers
     
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